This invention relates to an image processing apparatus and an image processing system adapted for shifting pixel value between pixels to constitute image texture.
In recent years, there have been popularized printers, digital copy machines, facsimiles and the like as image processing apparatuses and/or systems adapted for reading a picture image of a color manuscript by image input means, e.g., a color scanner, etc. to input such image to copy this image, or to carry out a predetermined image processing such as picture quality adjustment, image transform processing or editing processing, etc. with respect to such input image thereafter to provide hard copy output of that image onto paper by image output means such as a color printer of the electric photographic system, etc.
In such image processing apparatuses, quality and stability of image to be copied or duplicated are important, and gradation representation is required for the purpose of representing a photograph, etc. In order to carry out such gradation representation, various systems such as dither system, etc. have been developed. In the field of the hard copies , stripe pattern systems have been widely used.
A stripe pattern system is the system of representing, as shown in FIG. 19, ink (black) portions 160 distributed in units of pixels 150 in ordinary natural image by texture consisting of sets of longitudinal lines periodically arranged in a column manner in which such ink portions 160 are collected at the same main scanning position, wherein gradations are represented by widths of respective longitudinal lines. In this stripe pattern system, since ink portions of respective pixels are continuous in the longitudinal direction, frequency of ink (dot) modulation pattern is low so that resolution is lowered, whereas stable recording is advantageously carried out.
The example where modulation frequency in the stripe pattern system is further lowered is shown in FIG. 20. In this case, ink portions 160 existing in the main scanning direction are moved between pixels 150 so that dots are connected. In this example, with three pixels being as unit, left and right ink portions are concentrated in the state where central pixel is caused to be center. The method of FIG. 20 is more popular than the method of FIG. 19. This also results from the fact that width per one dot is 42 μm at 600 dpi from the requirement of realization of high picture quality, whereas laser beam diameter is about 70 μm, so processing of one pixel unit is actually impossible.
As shown in FIG. 21, as a method of generating stripe pattern, there is popular a method of generating reference signal 210 having periodical property like triangular wave, obtaining the relationship in magnitude between the reference signal 210 and pixel value 220 in the form of electric signal by means of a comparator in analog manner, then attaching ink only onto the portions of larger pixel value. In the case of a laser printer, since ink attached region is substantially equal to the laser light emitted region, the above-mentioned method can be realized by preparing circuit in which laser is light-emitted when value obtained by subtracting reference signal value from pixel value is positive.
On the other hand, in the case of carrying out color recording, color recording is conducted in the state where plates (printing blocks) of four colors of CMYK having such stripe pattern structure are overlapped. When stripe patterns of respective plates are exactly the same, interference takes place between stripe patterns so that Moire fringes occur. In order to prevent such interference, it is effective to employ a method of varying, every plates, angle or phase of stripe pattern to allow frequency of the interference fringes to be unnoticeable high frequency.
The method shown in FIGS. 22A and 22B is proposed in the Japanese Patent Application Laid Open No. 230163/1987 as an example of such a method, wherein phases of reference signal 210 of the first line shown in FIG. 22A and reference signal 230 of the second line shown in FIG. 22B are shifted every line.
However, such a method requires a complicated and large scale circuit for the purpose of generating periodical analog reference signal.
Furthermore, at the time of preparing stripe patterns of different angles for every plates of respective colors in color recording, plural periodical electric signals having various phases corresponding to angles must be prepared, so the circuit scale is further enlarged.
Furthermore, since such periodical electric signal is analog signal, anti-noise tolerance is weak (low). In the case of color recording, it is also difficult to adjust, with good accuracy, phase differences between plural periodical electric signals, resulting in the problem that smooth stripe pattern having angle cannot be generated. In addition, in dependency upon picture signal, there take place gaps 240 between stripe patterns as shown in FIG. 23, and dot modulation frequency in the main scanning direction is caused to be rather high. Thus, stable recording cannot be disadvantageously carried out.
In order to eliminate the drawbacks of the above-described analog circuit, it has become possible to carry out the above-described processing by digital circuit by using the digital circuit technologies which have been developed in recent years. Namely, there is a method in which, in place of analog reference signals, the portion within pixel is finely divided by high frequency clock to obtain, in a pseudo manner, then reference signals by means of counter circuit to compare them with picture signals. As compared to the analog system, this method is tolerable to electric noise, and permits the circuit scale to be compact. However, even if such a method is employed, in the case of image as shown in FIG. 23, it is not inevitable that gaps 240 take place.
As a method of solving gap of such picture signal, there has been proposed a technology in which picture image is divided into blocks 310 as shown in FIG. 24 to shift pixel 150 to shifted pixel 320 in accordance with pixel position within the block 310 to form cluster of dots within the blocks to connect clusters of dots of the respective blocks to generate stripe pattern (Japanese Patent Application Laid Open No. 41473/1999). With this method, however, since pixel is shifted only by information of pixel position irrespective of pixel value, phase ph can be controlled only in one pixel units as shown in FIG. 25. As a result, there is the problem that it is impossible to freely generate arbitrary screen angle.
Moreover, when attempt is made to represent or express, by stripe pattern, low density portion in which ratio of pixels where dots exist is low as shown in FIG. 26, i.e., highlight portion, width W of the stripe pattern becomes extremely small. As a result, density becomes unstable in the image recording characteristic, giving rise to rough feeling.
In view of the above, there is also devised a technology in which, with respect to the highlight portion, as shown in FIG. 27, blocks of units larger than block unit constituting the stripe pattern are constituted from plural pixels existing in the main scanning direction to collect pixel values within respective blocks into one shifted pixel 330 to prepare cluster of dots, and to shift that dot cluster position every line to thereby carry out recording by pattern like dot (Japanese Patent Application Laid Open No. 205603/1999). With this method, however, since dots are collected at one portion, dots can be formed only at the central portions of pixels. As a result, the degree of freedom of phase of dots within pixels in the main scanning direction is low. Therefore, there is no problem in the case of screen angle 340 in which gradient is 1/N (N=3 in FIG. 27) as shown in FIG. 27. However, in the case of screen angle 350 except for 1/N such that screen angle gradient is 3/6 as shown in FIG. 28, intervals of dots become thin, giving feeling of noise to an observer.
As described above, in the technologies which are known at present, e.g., in the method of generating recording device drive pulse by comparison between periodical electric signal and pixel data to carry out recording of the stripe pattern structure, gaps are formed between stripe patterns, and dot pattern is caused to have high frequency, resulting in the problem that recording becomes unstable. Further, in the analog circuit, there is the problem that noise is large. On the other hand, also in the method by shift image, there is the problem that it is impossible to take arbitrary screen angle with respect to stripe pattern.
In addition, in the method of periodically collecting pixels at one pixel to form dots to record highlight portion, there was the problem that dot pattern having arbitrary gradient cannot be spatially uniformly generated, giving rise to noise.